High performance GeSi avalanche photodiode operating beyond Ge bandgap limits

ABSTRACT

Avalanche photodiodes (APDs) having at least one top stressor layer disposed on a germanium (Ge) absorption layer are described herein. The top stressor layer can increase the tensile strain of the Ge absorption layer, thus extending the absorption of APDs to longer wavelengths beyond 1550 nm. In one embodiment, the top stressor layer has a four-layer structure, including an amorphous silicon (Si) layer disposed on the Ge absorption layer; a first silicon dioxide (SiO 2 ) layer disposed on the amorphous Si layer, a silicon nitride (SiN) layer disposed on the first SiO 2  layer, and a second SiO 2  layer disposed on the SiN layer. The Ge absorption layer can be further doped by p-type dopants. The doping concentration of p-type dopants is controlled such that a graded doping profile is formed within the Ge absorption layer to decrease the dark currents in APDs.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is continuation-in-part of U.S. patent application Ser.No. 13/604,911, filed on Sep. 6, 2012 and claiming the priority benefitof U.S. patent application Ser. No. 61/688,059, filed on May 5, 2012.The aforementioned applications are incorporated by reference in theirentirety.

TECHNICAL FIELD

The present disclosure relates to photosensitive devices. Moreparticularly, the present disclosure relates to an avalanche photodiode.

BACKGROUND

An avalanche photodiode (APD) is a type of photosensitive semiconductordevice in which light is converted to electricity due to thephotoelectric effect coupled with electric current multiplication as aresult of avalanche breakdown. APDs differ from conventional photodiodesin that incoming photons internally trigger a charge avalanche in APDs,thus APDs can measure light of even lower level and are widely used inlong-distance optical communications and optical distance measurementwhere high sensitivity is needed.

Germanium/Silicon (GeSi) APDs combine the characteristic of excellentoptical absorption of Ge at telecommunication wavelength with thecharacteristic of outstanding carrier multiplication properties of Si.The use of Ge allows the extension of the spectral response of GeSi APDsto longer wavelengths, up to 1550 nm. However, the absorption of bulk Geceases at 1550 nm at room temperature, which is limited by its bandgapin Gamma band. Since there is a requirement for the optical band inoptical communication systems to cover a wide wavelength range, from1260 nm to 1620 nm, the longer wavelength limitation of opticalabsorption of Ge is a main reason restricting the wide application ofGeSi APDs in optical communication systems. Therefore, there is a needto extend the absorption of Ge to longer wavelengths above 1550 nm.

One of the parameters that impact the applicability and usefulness ofAPDs is dark current. Dark current is a relatively small electriccurrent that flows through a photosensitive device, such as aphotodiode, even when no photons are entering the photosensitive device.Dark current is one of the major sources of noise in photosensitivedevices. Consequently, dark current is a limiting factor for GeSi APDsin high-speed optical communication applications. Therefore, there is aneed to reduce the dark current to achieve high performance in APDs.

SUMMARY

The present disclosure provides APDs having at least one top stressorlayer disposed on the light absorption layer. The top stressor layer canincrease the tensile strain of the absorption layer. As a result, theabsorption layer can absorb light with wavelengths beyond its opticalbandgap. The absorption layer can be further doped with p-type dopants.The doping concentration of the p-type dopants is controlled such that agraded doping profile is formed within the absorption layer to decreasethe dark currents of APDs.

According to one aspect, an avalanche photodiode (APD) may include asilicon-based substrate, e.g., a silicon substrate or asilicon-on-insulator (SOI) substrate, with a buried oxide (BOX) layer.The substrate may have a first side and a second side opposite the firstside. At least one bottom stressor layer may be disposed on the secondside of the substrate and adjacent to the BOX layer. The APD may alsoinclude a multi-layer structure disposed on the first side of thesubstrate. The multi-layer structure may include at least one topstressor layer coupled to at least one metal contact of a firstelectrical polarity and a germanium (Ge) absorption layer on which theat least one top stressor layer is disposed. The at least one topstressor layer may be configured to increase a tensile strain in the Geabsorption layer such that absorption of the Ge absorption layer between1550 nm and 1650 nm is increased.

In some embodiments, the at least one top stressor layer may furtherinclude an amorphous silicon layer, a first silicon dioxide (SiO₂) layerdisposed on the amorphous Si layer, a silicon nitride (SiN) layerdisposed on the first SiO₂ layer, and a second SiO₂ layer disposed onthe SiN layer.

In some embodiments, the Ge absorption layer may include Ge,germanium-silicon (GeSi), or silicon-germanium-carbon (SiGeC).

In some embodiments, the charge layer may include p-type Si, p-typeGeSi, or p-type SiGeC.

In some embodiments, the multiplication layer may include intrinsic Sior lightly doped n-type Si.

In some embodiments, the contact layer may include n-type Si.

In some embodiments, the at least one bottom stressor layer may functionas a reflection layer and may be configured to increase a tensile strainin the Ge absorption layer.

In some embodiments, the multi-layer structure may further include acharge layer on which the Ge absorption layer is disposed, amultiplication layer on which the charge layer is disposed, and acontact layer on which the multiplication layer is disposed, the contactlayer coupled to at least one metal contact of a second electricalpolarity opposite to the first electrical polarity.

In some embodiments, the Ge absorption layer may further include p-typedopants. A doping concentration of the p-type dopants may be controlledsuch that a graded doping profile of the p-type dopants is formed withinthe Ge absorption layer. The p-type dopant may include gallium (Ga) orboron (B).

In some embodiments, the bottom stressor layer may include a metal layerincluding aluminum, titanium, gold, silver, nickel, cobalt, platinum, ortungsten.

According to another aspect, an APD may include a silicon-basedsubstrate, e.g., a silicon substrate or a SOI substrate, with a BOXlayer. The substrate may have a first side and a second side oppositethe first side. At least one bottom stressor layer may be disposed onthe second side of the substrate and adjacent to the BOX layer. The APDmay also include a multi-layer structure disposed on the first side ofthe substrate. The multi-layer structure may include at least one topstressor layer coupled to at least one metal contact of a firstelectrical polarity, a Ge absorption layer on which the at least one topstressor layer is disposed, a charge layer on which the Ge absorptionlayer is disposed, a multiplication layer on which the charge layer isdisposed, and a contact layer on which the multiplication layer isdisposed, the contact layer coupled to at least one metal contact of asecond electrical polarity opposite to the first electrical polarity.The at least one top stressor layer may be configured to increase atensile strain in the Ge absorption layer such that absorption of the Geabsorption layer between 1550 nm and 1650 nm is increased.

In some embodiments, the at least one top stressor layer may furtherinclude an amorphous silicon layer, a first SiO₂ layer disposed on theamorphous Si layer, a SiN layer disposed on the first SiO₂ layer, and asecond SiO₂ layer disposed on the SiN layer.

In some embodiments, the charge layer may include p-type Si, p-typeGeSi, or p-type SiGeC, wherein the multiplication layer comprisesintrinsic Si or lightly doped n-type Si. The contact layer may includen-type Si.

In some embodiments, the at least one bottom stressor layer may functionas a reflection layer and may be configured to increase a tensile strainin the Ge absorption layer.

In some embodiments, the Ge absorption layer may further include p-typedopants. A doping concentration of the p-type dopants may be controlledsuch that a graded doping profile of the p-type dopants is formed withinthe Ge absorption layer. The p-type dopants may include Ga or B.

In some embodiments, the bottom stressor layer may include a metal layerincluding aluminum, titanium, gold, silver, nickel, cobalt, platinum, ortungsten.

According to yet another aspect, an APD may include a silicon-basedsubstrate, e.g., a silicon substrate or a SOI substrate, with a BOXlayer. The substrate may have a first side and a second side oppositethe first side. At least one bottom stressor layer may be disposed onthe second side of the substrate and adjacent to the BOX layer. The APDmay also include a multi-layer structure disposed on the first side ofthe substrate. The multi-layer structure may include at least one topstressor layer coupled to at least one metal contact of a firstelectrical polarity, a Ge absorption layer doped with p-type dopants onwhich the at least one top stressor layer is disposed. A dopingconcentration of the p-type dopants may be controlled such that a gradeddoping profile of the p-type dopants is formed within the Ge absorptionlayer. The multi-layer structure may further include a charge layer onwhich the Ge absorption layer is disposed, a multiplication layer onwhich the charge layer is disposed, and a contact layer on which themultiplication layer is disposed, the contact layer coupled to at leastone metal contact of a second electrical polarity opposite to the firstelectrical polarity. The at least one bottom stressor layer may functionas a reflection layer and is configured to increase a tensile strain inthe Ge absorption layer.

In some embodiments, the Ge absorption layer comprises Ge, GeSi, orSiGeC. The p-type dopant may include Ga or B. The at least one topstressor layer may further include an amorphous silicon layer, a firstSiO₂ layer disposed on the amorphous Si layer, a SiN layer disposed onthe first SiO₂ layer, and a second SiO₂ layer disposed on the SiN layer.

In some embodiments, the bottom stressor layer may include a metal layerincluding aluminum, titanium, gold, silver, nickel, cobalt, platinum, ortungsten.

These and other features, aspects, and advantages of the presentdisclosure will be explained below with reference to the followingfigures. It is to be understood that both the foregoing generaldescription and the following detailed description are by examples, andare intended to provide further explanation of the present disclosure asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present disclosure, and are incorporated in andconstitute a part of this specification. The drawings illustrateembodiments of the present disclosure and, together with thedescription, serve to explain the principles of the present disclosure.The drawings may not necessarily be in scale so as to better presentcertain features of the illustrated subject matter.

FIG. 1A is a cross-sectional view of an APD in accordance with someembodiments of the present disclosure.

FIG. 1B is a cross-sectional view of an APD in accordance with anexemplary embodiment of the present disclosure.

FIG. 2A is a cross-sectional view of an APD in accordance with someembodiments of the present disclosure.

FIG. 2B is a cross-sectional view of an APD in accordance with anexemplary embodiment of the present disclosure.

FIG. 3A is a cross-sectional view of an APD in accordance with someembodiments of the present disclosure.

FIG. 3B is a cross-sectional view of an APD in accordance with anexemplary embodiment of the present disclosure.

FIG. 4A is a cross-sectional view of an APD in accordance with someembodiments of the present disclosure.

FIG. 4B is a cross-sectional view of an APD in accordance with anexemplary embodiment of the present disclosure.

FIG. 5 is a cross-sectional view of an APD with both top and bottomstressor layers in accordance with an exemplary embodiment of thepresent disclosure.

FIG. 6 is a graph comparing Raman spectra of a bulk Ge layer and a Gelayer having top stress layers in accordance with the presentdisclosure.

FIG. 7 is a graph comparing absorption spectra of a bulk Ge layer and aGe layer having top stress layers in accordance with the presentdisclosure.

FIG. 8 is a graph of simulation results of stress in Ge with and withouta bottom stressor layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

The present disclosure provides avalanche photodiodes (APDs) having topstressor layers disposed on an absorption layer that can increase thetensile strains of the absorption layer. As a result, the opticalabsorption in wavelengths beyond the optical bandgap of the absorptionlayer is enhanced to achieve high device performance. Illustrative APDsof the present disclosure are schematically shown in cross-sectionalviews in FIGS. 1-4. FIGS. 1-4 are not drawn to scale and are provided toconvey the concept of the various embodiments of the present disclosure.

Exemplary Embodiments

FIG. 1A is a cross-sectional view of an APD 100 in accordance with anembodiment of the present disclosure. Referring to FIG. 1, the APD 100may comprise a substrate 110 and a multi-layer structure 120 disposed onthe substrate 110. The multi-layer structure 120 may comprise: a topstressor layer 130 electrically coupled to one or more one first-typemetal contacts 135 of a first electrical polarity, an absorption layer140 on which the top stressor layer 130 is disposed, a charge layer 150on which the absorption layer 140 is disposed, a multiplication layer160 on which the charge layer 150 is disposed, and a contact layer 170on which the multiplication layer 160 is disposed. One or moresecond-type metal contacts 175 of a second electrical polarity areelectrically coupled to the contact layer 170. The second electricalpolarity is opposite to the first electrical polarity. For example, theone or more first-type metal contacts 135 are p-type and the one or moresecond-type metal contacts 175 are n-type, or vice versa. The APD 100may further comprise an oxide coating 180 that covers the multi-layerstructure 120.

The top stressor layer 130 increases the tensile strain of theabsorption layer 140, thus greatly enhancing optical absorption inwavelengths beyond the optical bandgap of the absorption layer 140. Thetop stressor layer 130 also serves as an anti-reflection layer toimprove the quantum efficiency of the APD 100. The top stressor layer130 can be a single-layer or multi-layer structure. In one embodiment,the top stressor layer 130 has a multi-layer structure comprising fourlayers, including an amorphous silicon (Si) layer 1301 disposed on theabsorption layer 140; a first silicon dioxide (SiO₂) layer 1302 disposedon the amorphous Si layer 1301; a silicon nitride (SiN) layer 1303disposed on the first SiO₂ layer 1302; and a second SiO₂ layer 1304disposed on the SiN layer 1303. The amorphous Si layer is electricallycoupled to the one or more first-type metal contracts 135.

In one embodiment, the absorption layer 140 includes germanium (Ge),germanium-silicon (GeSi), or silicon-germanium-carbon (SiGeC). In oneembodiment, the charge layer 150 includes p-type Si, p-type GeSi, orp-type SiGeC. In one embodiment, the multiplication layer 160 includesintrinsic Si or lightly doped n-type Si. In one embodiment, the contactlayer 170 includes n-type Si. In one embodiment, the substrate 110includes a Si substrate or a silicon-on-insulator (SOI) substrate.

FIG. 1B illustrates an exemplary embodiment of the APD 100. In theillustrated embodiment, the top stressor layer 130 is electricallycoupled to two p-type metal contacts 135, the absorption layer 140 is aGe absorption layer, the charge layer 150 is a p-type Si layer, themultiplication layer 160 is a Si multiplication layer, and the contactlayer 170 is an n-type Si layer. The contact layer 170 is electricallycoupled to two n-type metal contacts 175.

FIG. 2A is a cross-sectional view of an APD 200 in accordance with anembodiment of the present disclosure. Referring to FIG. 2A, the APD 200may comprise a substrate 210, a multi-layer structure 220 disposed on afirst surface of the substrate 210, and an anti-reflection layer 290disposed on a second surface of the substrate 210 opposite to the firstsurface. The multi-layer structure 220 may comprise: a top stressorlayer 230 electrically coupled to one or more first-type metal contacts235 of a first electrical polarity, an absorption layer 240 on which thetop stressor layer 230 is disposed, a charge layer 250 on which theabsorption layer 240 is disposed, a multiplication layer 260 on whichthe charge layer 250 is disposed, and a contact layer 270 on which themultiplication layer 260 is disposed. One or more second-type metalcontacts 275 of a second electrical polarity are electrically coupled tothe contact layer 270. The second electrical polarity is opposite to thefirst electrical polarity. For example, the one or more first-type metalcontacts 235 are p-type and the one or more second-type metal contacts275 are n-type, or vice versa. The APD 200 may further comprise an oxidecoating 280 that covers the multi-layer structure 220.

The anti-reflection layer 290 can be a single-layer or multi-layerstructure. In one embodiment, the anti-reflection layer 290 is a singleSiO₂ layer. In another embodiment, the anti-reflection layer 290 hasthree layers, including a SiN layer disposed between two SiO₂ layers.

The top stressor layer 230 increases the tensile strain of theabsorption layer 240, thus greatly enhancing optical absorption inwavelengths beyond the optical bandgap of the absorption layer 240. Thetop stressor layer 230 also serves as an anti-reflection layer toimprove the quantum efficiency of the APD 200. The top stressor layer230 can be a single-layer or multi-layer structure. In one embodiment,the top stressor layer 230 has a multi-layer structure comprising fourlayers, including an amorphous Si layer 2301 disposed on the absorptionlayer 240; a first SiO₂ layer 2302 disposed on the amorphous Si layer2301; a SiN layer 2303 disposed on the first SiO₂ layer 2302; and asecond SiO₂ layer 2304 disposed on the SiN layer 2303. The amorphous Silayer is electrically coupled to the one or more first-type metalcontracts 235.

In one embodiment, the absorption layer 240 includes Ge, GeSi, or SiGeC.In one embodiment, the charge layer 250 includes p-type Si p-type GeSi,or p-type SiGeC. In one embodiment, the multiplication layer 260includes intrinsic Si, or lightly doped n-type Si. In one embodiment,the contact layer 270 includes n-type Si. In one embodiment, thesubstrate 210 includes a Si substrate or an SOI substrate.

FIG. 2B illustrates an exemplary embodiment of the APD 200. In theillustrated embodiment, the top stressor layer 230 is electricallycoupled to two p-type metal contacts 235, the absorption layer 240 is aGe absorption layer, the charge layer 250 is a p-type Si layer, themultiplication layer 260 is a Si multiplication layer, and the contactlayer 270 is an n-type Si layer. The contact layer 270 is electricallycoupled to two n-type metal contacts 275.

In comparison with the APD 100, the APD 200 in accordance with FIGS.2A-2B further comprises the anti-reflection layer 290. Incoming Light ofoptical signals may be illuminated from the side of the anti-reflectionlayer 290 to enter into the APD 200. Thus, the anti-reflection layer 290helps avoid optical loss at the incident surface 295. Moreover, whenoperating under this bottom illumination condition, the opticalabsorption of the APD 200 can be further increased. Due to the presenceof the highly reflective top stressor layer 230, a major portion of theoptical signals that has already passed through the absorption layer 240will be reflected back into the absorption layer 240, thus effectivelyincreasing optical absorptions of the absorption layer 240, especiallyfor those wavelengths beyond the bandgap limits of the absorption layer240.

FIG. 3A is a cross-sectional view of an APD 300 in accordance with anembodiment of the present disclosure. Referring to FIG. 3A, the APD 300may comprise a substrate 310 and a multi-layer structure 320 disposed onthe substrate 310. The multi-layer structure 320 may comprise: a topstressor layer 330 electrically coupled to one or more first-type metalcontacts 335 of a first electrical polarity, an absorption layer 340doped with first-type dopants and on which the top stressor layer 330 isdisposed, a charge layer 350 on which the absorption layer 340 isdisposed, a multiplication layer 360 on which the charge layer 350 isdisposed, and a contact layer 370 on which the multiplication layer 360is disposed. One or more second-type metal contacts 375 of a secondelectrical polarity are electrically coupled to the contact layer 370.The second electrical polarity is opposite to the first electricalpolarity. For example, the one or more first-type metal contacts 335 arep-type and the one or more second-type metal contacts 375 are n-type, orvice versa. The APD 300 may further comprise an oxide coating 380 thatcovers the multi-layer structure 320. The doping concentration of thefirst-type dopants in the absorption layer 340 is controlled such that agraded doping profile of the first-type dopants is formed within theabsorption layer 340. The graded doping profile of the first-typedopants is shown in FIG. 3A. For example, the first-type dopants arep-type dopants.

The top stressor layer 330 increases the tensile strain of theabsorption layer 340, thus greatly enhancing optical absorption inwavelengths beyond the optical bandgap of the absorption layer 340. Thetop stressor layer 330 also serves as an anti-reflection layer toimprove the quantum efficiency of the APD 300. The top stressor layer330 can be a single-layer or multi-layer structure. In one embodiment,the top stressor layer 330 has a multi-layer structure comprising fourlayers, including an amorphous Si layer 3301 disposed on the absorptionlayer 340; a first SiO₂ layer 3302 disposed on the amorphous Si layer3301; a SiN layer 3303 disposed on the first SiO₂ layer 3302; and asecond SiO₂ layer 3304 disposed on the SiN layer 3303. The amorphous Silayer is electrically coupled to the one or more first-type metalcontracts 335.

In one embodiment, the absorption layer 340 includes Ge, GeSi, or SiGeC.In one embodiment, the charge layer 350 includes p-type Si, p-type GeSi,or p-type SiGeC. In one embodiment, the multiplication layer 360includes intrinsic Si, or lightly doped n-type Si. In one embodiment,the contact layer 370 includes n-type Si. In one embodiment, thesubstrate 310 includes a Si substrate or an SOI substrate. In oneembodiment, the p-type dopants include gallium (Ga) or boron (B).

FIG. 3B illustrates an exemplary embodiment of the APD 300. In theillustrated embodiment, the top stressor layer 330 is electricallycoupled to two p-type metal contacts 335, the absorption layer 340 is ap-type Ge absorption layer, the charge layer 350 is a p-type Si layer,the multiplication layer 360 is a Si multiplication layer, and thecontact layer 370 is an n-type Si layer. The contact layer 370 iselectrically coupled to two n-type metal contacts 375.

In comparison with the APD 100, the APD 300 in accordance with FIGS.3A-3B has an undepleted absorption layer 340 with a graded dopingprofile for reducing the electrical field and dark current within theabsorption layer 340. The graded first-type doping of the absorptionlayer 340 can be achieved by in-situ doping or ion implantation. Thegraded doping profile of the first-type dopants formed in the absorptionlayer 340 can generate a built-in electrical field. This electricalfield is mainly dependent on doping gradients and is independent onexternal applied bias. For example, as shown in FIG. 3B, with a properdesign of the doping profile in the p-type Ge absorption layer 340, thebuilt-in electrical field can reach several kV/cm in the p-type Geabsorption layer 340, thus ensuring carriers drift velocities close tosaturation velocities. As a result, with extremely low dark current,GeSi APDs with an undepleted absorption layer can operate at a highspeed condition like conventional GeSi APDs.

Moreover, considering the electrical field inside the p-type Geabsorption layer 340, the built-in electrical field (several kV/cm) inthe APD 300 is much weaker than that of the conventional GeSi APDs (−100kV/cm). Since dark currents in GeSi APDs are mainly depended on theelectrical field inside the Ge absorption layer, GeSi APDs with anundepleted absorption layer can significantly reduce dark currents inGeSi APDs.

FIG. 4A is a cross-sectional view of an APD 400 in accordance with anembodiment of the present disclosure. Referring to FIG. 4A, the APD 400may comprise a substrate 410 and a multi-layer structure 420 disposed onthe substrate 410. The substrate 410 is a silicon-based substrate, e.g.,a silicon substrate or a SOI substrate, with a buried oxide (BOX) layer415. The multi-layer structure 420 may comprise: a top stressor layer430 electrically coupled to one or more first-type metal contacts 435 ofa first electrical polarity, an absorption layer 440 on which the topstressor layer 430 is disposed, a charge layer 450 on which theabsorption layer 440 is disposed, a multiplication layer 460 on whichthe charge layer 450 is disposed, and a contact layer 470 on which themultiplication layer 460 is disposed. One or more second-type metalcontacts 475 of a second electrical polarity are electrically coupled tothe contact layer 470. The second electrical polarity is opposite to thefirst electrical polarity. For example, the one or more first-type metalcontacts 435 are p-type and the one or more second-type metal contacts475 are n-type, or vice versa. The APD 400 may further comprise an oxidecoating 480 that covers the multi-layer structure 420.

The top stressor layer 430 increases the tensile strain of theabsorption layer 440, thus greatly enhancing optical absorption inwavelengths beyond the optical bandgap of the absorption layer 440. Thetop stressor layer 430 also serves as an anti-reflection layer toimprove the quantum efficiency of the APD 400. The top stressor layer430 can be a single-layer or multi-layer structure. In one embodiment,the top stressor layer 430 has a multi-layer structure comprising fourlayers, including an amorphous Si layer 4301 disposed on the absorptionlayer 440; a first SiO₂ layer 4302 disposed on the amorphous Si layer4301; a SiN layer 4303 disposed on the first SiO₂ layer 4302; and asecond SiO₂ layer 4304 disposed on the SiN layer 4303. The amorphous Silayer is electrically coupled to the one or more first-type metalcontracts 435.

The absorption layer 440 can be an intrinsic semiconductor layer or asemiconductor layer doped with first-type dopants. The dopingconcentration of the first-type dopants is controlled such that a gradeddoping profile of the first-type dopants is formed within the absorptionlayer 440. For example, the first-type dopants are p-type dopants.

In one embodiment, the absorption layer 440 includes Ge, GeSi, or SiGeC.In one embodiment, the charge layer 450 includes p-type Si, p-type GeSi,or p-type SiGeC. In one embodiment, the multiplication layer 460includes intrinsic Si, or lightly doped n-type Si. In one embodiment,the contact layer 470 includes n-type Si. In one embodiment, thesubstrate 410 includes a Si substrate or an SOI substrate. In oneembodiment, the p-type dopants include gallium (Ga) or boron (B).

FIG. 4B illustrates an exemplary embodiment of the APD 400. In theillustrated embodiment, the substrate 410 is a SOI substrate with BOX415, the top stressor layer 430 is electrically coupled to two p-typemetal contacts 435, the absorption layer 440 is a Ge absorption layer,the charge layer 450 is a p-type Si layer, the multiplication layer 460is a Si multiplication layer, and the contact layer 470 is an n-type Silayer. The contact layer 470 is electrically coupled to two n-type metalcontacts 475.

The APD 400 in accordance with FIGS. 4A-4B can operate under lateralincident illumination condition like a waveguide device. The light beamis incident laterally at the junction of the absorption layer 440 andthe charge layer 450 of the APD 400. Normally, the dark currents in APDsare proportional to the size of the area of the absorption layer. Sincea waveguide device typically has a much smaller size than a normalincident device, the design of the present disclosure can reduce darkcurrents in GeSi APDs. In addition, a waveguide device according to thepresent disclosure also has a broader absorption coverage resulted fromits lateral incident illumination and a better bandwidth resulted fromits smaller capacitance. As a result, the device performance can begreatly enhanced.

FIG. 5 is a cross-sectional view of an APD 500 in accordance with anexemplary embodiment of the present disclosure. Referring to FIG. 5, theAPD 500 may comprise a substrate 510 and a multi-layer structure 520disposed on the substrate 510. The substrate 510 includes a buried oxide(BOX) layer 515. The multi-layer structure 520 may comprise: a topstressor layer 530 electrically coupled to one or more one first-typemetal contacts 535 of a first electrical polarity, an absorption layer540 on which the top stressor layer 530 is disposed, a charge layer 550on which the absorption layer 540 is disposed, a multiplication layer560 on which the charge layer 550 is disposed, and a contact layer 570on which the multiplication layer 560 is disposed. One or moresecond-type metal contacts 575 of a second electrical polarity areelectrically coupled to the contact layer 570. The second electricalpolarity is opposite to the first electrical polarity. For example, theone or more first-type metal contacts 535 are p-type and the one or moresecond-type metal contacts 575 are n-type, or vice versa. The APD 500may further comprise an oxide coating 580 that covers the multi-layerstructure 520.

The top stressor layer 530 increases the tensile strain of theabsorption layer 540, thus greatly enhancing optical absorption inwavelengths beyond the optical bandgap of the absorption layer 540. Thetop stressor layer 530 also serves as an anti-reflection layer toimprove the quantum efficiency of the APD 500. The top stressor layer530 can be a single-layer or multi-layer structure. In one embodiment,the top stressor layer 530 has a multi-layer structure comprising fourlayers, including an amorphous silicon (Si) layer 5301 disposed on theabsorption layer 540; a first silicon dioxide (SiO₂) layer 5302 disposedon the amorphous Si layer 5301; a silicon nitride (SiN) layer 5303disposed on the first SiO₂ layer 5302; and a second SiO₂ layer 5304disposed on the SiN layer 5303. The amorphous Si layer is electricallycoupled to the one or more first-type metal contracts 535.

In one embodiment, the absorption layer 540 includes germanium (Ge),germanium-silicon (GeSi), or silicon-germanium-carbon (SiGeC). In oneembodiment, the charge layer 550 includes p-type Si, p-type GeSi, orp-type SiGeC. In one embodiment, the multiplication layer 560 includesintrinsic Si or lightly doped n-type Si. In one embodiment, the contactlayer 570 includes n-type Si. In one embodiment, the substrate 510includes a Si substrate or a silicon-on-insulator (SOI) substrate.

APD 500 is a high-performance GeSi avalanche photodiode operating beyondGe bandgap limits, and has enhanced tensile strain in the Ge absorptionlayer 540 as well as enhanced Ge absorption of wavelengths beyond thebandgap of bulk Ge. Compared to APD 100, APD 200, APD 300 and APD 400,APD 500 includes a bottom stressor layer 590 to further enhance Geabsorption of APD 500, especially for the wavelengths beyond the bandgapof bulk Ge. The bottom stressor layer 590 also enhances tensile strainin the Ge absorption layer 540 and functions as a bottom reflectionlayer. As shown in FIG. 5, the bottom stressor layer 590 is deposited ona bottom side of the BOX layer 515. In simulation, the bottom stressorlayer 590 may increase the tensile strain in the Ge absorption layer 540by 20-30% depending on the depth in the Ge absorption layer 540. Inother words, the bottom stressor layer 590 is configured to increase thetensile strain in the Ge absorption layer 540 by at least 20%.

In one embodiment as illustrated in FIG. 5, the top stressor layer 530is electrically coupled to two p-type metal contacts 535, the absorptionlayer 540 is a Ge absorption layer, the charge layer 550 is a p-type Silayer, the multiplication layer 560 is a Si multiplication layer, andthe contact layer 570 is an n-type Si layer. The contact layer 570 iselectrically coupled to two n-type metal contacts 575. The top stressorlayer 530 enhances the tensile strain of the Ge absorption layer 540 andserves as an anti-reflection layer of APD 500.

In one embodiment, the bottom stressor layer 590 may be a metal layer,and may comprise any of aluminum, titanium, gold, silver, nickel,cobalt, platinum, tungsten, etc.

In one embodiment, the bottom stressor layer 590 may be a single-layerstructure or a multiple-layers structure.

In one embodiment, the bottom stressor layer 590 may be fabricated by aprocess including a number of steps. First, the thickness of the Sisubstrate 510 is reduced to some target value by backside grinding.Next, the Si substrate 510 is etched (beneath APD device region) and theetching stops at the bottom surface of BOX layer 515. Subsequently, thebottom stressor layer 590 is deposited by evaporation or other suitablemethods.

Exemplary Test Results

Raman spectra and absorption spectra of a bulk Ge layer and a Ge layerhaving top stressor layers in accordance with the present disclosurewere measured to study the effects of the top stressor layer on theoptical properties of Ge. In this study, the top stressor layer has afour-layer structure, including an amorphous Si layer disposed on the Geabsorption layer; a first SiO₂ layer disposed on the amorphous Si layer;a SiN layer disposed on the first SiO₂ layer; and a second SiO₂ layerdisposed on the SiN layer.

FIG. 6 shows a graph 600 comparing Raman spectra of a bulk Ge layer anda Ge layer having top stressor layers in accordance with the presentdisclosure. The Ge Raman spectra peaks are at 300.4 cm⁻¹ and 299.3 cm⁻¹for the bulk Ge layer and the Ge layer having top stressor layers,respectively. The difference in Raman spectra peaks indicates that thetop stressor layers can increase tensile strain inside the Ge layer.

FIG. 7 shows a graph 700 comparing absorption spectra of a bulk Ge layerand a Ge layer having top stressor layers in accordance with the presentdisclosure. The Ge layer having top stressor layers has much higherabsorption coefficient between 1500 nm to 1600 nm than those of the bulkGe layer. The absorption spectra clearly show that the bulk Ge layercannot efficiently absorb the light with wavelengths beyond 1550 nm,while the Ge layer with top stressor layers not only extends theabsorption edge to 1600 nm but also greatly increase the absorptioncoefficient at 1550 nm. For example, as shown in graph 700, the topstressor layer in APD 100, APD 200, APD 300, APD 400 and APD 500 isconfigured to increase a tensile strain in the Ge absorption layer suchthat absorption of the Ge absorption layer between 1550 nm and 1650 nmis increased.

FIG. 8 shows a graph 800 of simulation results of stress in Ge with andwithout a bottom stressor layer. The simulation results prove that abottom stressor layer can enlarge stress tensor in the Ge absorptionlayer. This means the Ge absorption layer is under a larger tensilestrain and so the Ge absorption layer has better absorption especiallyfor wavelengths beyond the bandgap of the bulk Ge of the Ge absorptionlayer. As shown in graph 800, the bottom stressor layer is configured toincrease a tensile strain in the Ge absorption layer such that, fordevice with a bottom stressor layer similar to bottom stressor layer 590as in APD 500, the simulated stress in the Ge absorption layer is largerthan 3.05×10⁸ (a.u., or arbitrary unit). For device without any bottomstressor layer, the simulated stress in the Ge absorption layer is in arange of 2.2 to 2.9×10⁸ (a.u.). These simulation results prove that thebottom stressor layer can apply larger stress to the Ge absorption layerand enhance Ge absorption of wavelength beyond Ge bandgap.

In one embodiment, besides using a metal layer for the bottom stressorlayer 590, other layers such as a silicide layer and/or a dielectriclayer may also be applied to realize stress enhancement.

Additional Notes

Although some embodiments are disclosed above, they are not intended tolimit the scope of the present disclosure. It will be apparent to thoseskilled in the art that various modifications and variations can be madeto the disclosed embodiments of the present disclosure without departingfrom the scope or spirit of the present disclosure. In view of theforegoing, the scope of the present disclosure shall be defined by thefollowing claims and their equivalents.

What is claimed is:
 1. An avalanche photodiode, comprising: asilicon-based substrate with a buried oxide (BOX) layer, the substratehaving a first side and a second side opposite the first side; at leastone bottom stressor layer disposed on the second side of the substrateand in contact with the BOX layer, with a cavity formed underneath theat least one bottom stressor layer; and a multi-layer structure disposedon the first side of the substrate, comprising: at least one topstressor layer including an amorphous silicon (Si) layer, the at leastone top stressor layer coupled to at least one metal contact of a firstelectrical polarity; and a germanium (Ge) absorption layer on which theat least one top stressor layer is disposed such that the amorphous Silayer is in direct contact with the Ge absorption layer, wherein the atleast one top stressor layer is configured to increase a tensile strainin the Ge absorption layer such that absorption of the Ge absorptionlayer between 1550 nm and 1650 nm is increased.
 2. The avalanchephotodiode of claim 1, wherein the at least one top stressor layerfurther comprises: the amorphous silicon layer; a first silicon dioxide(SiO₂) layer disposed on the amorphous Si layer; a silicon nitride (SiN)layer disposed on the first SiO₂ layer; and a second SiO₂ layer disposedon the SiN layer.
 3. The avalanche photodiode of claim 1, wherein the Geabsorption layer comprises Ge, germanium-silicon (GeSi), orsilicon-germanium-carbon (SiGeC).
 4. The avalanche photodiode of claim1, further comprising: a charge layer on which the Ge absorption layeris disposed, wherein the charge layer comprises p-type Si, p-type GeSi,or p-type SiGeC.
 5. The avalanche photodiode of claim 4, furthercomprising: a multiplication layer on which the charge layer isdisposed, wherein the multiplication layer comprises intrinsic Si orlightly doped n-type Si.
 6. The avalanche photodiode of claim 5, furthercomprising: a contact layer on which the multiplication layer isdisposed, wherein the contact layer comprises n-type Si.
 7. Theavalanche photodiode of claim 1, wherein the at least one bottomstressor layer functions as a reflection layer and is configured toincrease a tensile strain in the Ge absorption layer.
 8. The avalanchephotodiode of claim 1, wherein the multi-layer structure furthercomprises: a charge layer on which the Ge absorption layer is disposed;a multiplication layer on which the charge layer is disposed; and acontact layer on which the multiplication layer is disposed, the contactlayer coupled to at least one metal contact of a second electricalpolarity opposite to the first electrical polarity.
 9. The avalanchephotodiode of claim 1, wherein the Ge absorption layer further comprisesp-type dopants, wherein a doping concentration of the p-type dopants iscontrolled such that a graded doping profile of the p-type dopants isformed within the Ge absorption layer, and wherein the p-type dopantcomprises gallium (Ga) or boron (B).
 10. The avalanche photodiode ofclaim 1, wherein the bottom stressor layer comprises a metal layerincluding aluminum, titanium, gold, silver, nickel, cobalt, platinum, ortungsten.
 11. An avalanche photodiode, comprising: a silicon-basedsubstrate with a buried oxide (BOX) layer, the substrate having a firstside and a second side opposite the first side; at least one bottomstressor layer disposed on the second side of the substrate and incontact with the BOX layer, with a cavity formed underneath the at leastone bottom stressor layer; and a multi-layer structure disposed on thefirst side of the substrate, comprising: at least one top stressor layerincluding an amorphous silicon (Si) layer, the at least one top stressorlayer coupled to at least one metal contact of a first electricalpolarity; a germanium (Ge) absorption layer on which the at least onetop stressor layer is disposed such that the amorphous Si layer is indirect contact with the Ge absorption layer; a charge layer on which theGe absorption layer is disposed; a multiplication layer on which thecharge layer is disposed; and a contact layer on which themultiplication layer is disposed, the contact layer coupled to at leastone metal contact of a second electrical polarity opposite to the firstelectrical polarity, wherein the at least one top stressor layer isconfigured to increase a tensile strain in the Ge absorption layer suchthat absorption of the Ge absorption layer between 1550 nm and 1650 nmis increased.
 12. The avalanche photodiode of claim 11, wherein the atleast one top stressor layer further comprises: the amorphous siliconlayer; a first silicon dioxide (SiO₂) layer disposed on the amorphous Silayer; a silicon nitride (SiN) layer disposed on the first SiO₂ layer;and a second SiO₂ layer disposed on the SiN layer.
 13. The avalanchephotodiode of claim 11, wherein the charge layer comprises p-type Si,p-type GeSi, or p-type SiGeC, wherein the multiplication layer comprisesintrinsic Si or lightly doped n-type Si, and wherein the contact layercomprises n-type Si.
 14. The avalanche photodiode of claim 11, whereinthe at least one bottom stressor layer functions as a reflection layer.15. The avalanche photodiode of claim 11, wherein the at least onebottom stressor layer is configured to increase a tensile strain in theGe absorption layer.
 16. The avalanche photodiode of claim 11, whereinthe Ge absorption layer further comprises p-type dopants, wherein adoping concentration of the p-type dopants is controlled such that agraded doping profile of the p-type dopants is formed within the Geabsorption layer, and wherein the p-type dopant comprises gallium (Ga)or boron (B).
 17. The avalanche photodiode of claim 11, wherein thebottom stressor layer comprises a metal layer including aluminum,titanium, gold, silver, nickel, cobalt, platinum, or tungsten.
 18. Anavalanche photodiode, comprising: a silicon-based substrate with aburied oxide (BOX) layer, the substrate having a first side and a secondside opposite the first side; at least one bottom stressor layerdisposed on the second side of the substrate and in contact with the BOXlayer, with a cavity formed underneath the at least one bottom stressorlayer; and a multi-layer structure disposed on the first side of thesubstrate, comprising: at least one top stressor layer including anamorphous silicon (Si) layer, the at least one top stressor layercoupled to at least one metal contact of a first electrical polarity; agermanium (Ge) absorption layer doped with p-type dopants on which theat least one top stressor layer is disposed, a doping concentration ofthe p-type dopants is controlled such that a graded doping profile ofthe p-type dopants is formed within the Ge absorption layer; a chargelayer on which the Ge absorption layer is disposed; a multiplicationlayer on which the charge layer is disposed; and a contact layer onwhich the multiplication layer is disposed, the contact layer coupled toat least one metal contact of a second electrical polarity opposite tothe first electrical polarity, wherein the at least one bottom stressorlayer functions as a reflection layer and is configured to increase atensile strain in the Ge absorption layer, and wherein the amorphous Silayer is in direct contact with the Ge absorption layer.
 19. Theavalanche photodiode of claim 18, wherein the Ge absorption layercomprises Ge, germanium-silicon (GeSi), or silicon-germanium-carbon(SiGeC), and wherein the p-type dopant comprises gallium (Ga) or boron(B), and wherein the at least one top stressor layer further comprises:the amorphous silicon layer; a first silicon dioxide (SiO₂) layerdisposed on the amorphous Si layer; a silicon nitride (SiN) layerdisposed on the first SiO₂ layer; and a second SiO₂ layer disposed onthe SiN layer.
 20. The avalanche photodiode of claim 18, wherein thebottom stressor layer comprises a metal layer including aluminum,titanium, gold, silver, nickel, cobalt, platinum, or tungsten.